1. Field of the invention
The present invention concerns a method of saving a wholly or partially three-axis stabilized satellite, in other words an attitude reacquisition procedure for returning the satellite or the relevant part thereof to its nominal service attitude.
2. Description of the prior art
The three axes with reference to which the attitude of a satellite (or part of a satellite) is stabilized are an axis directed towards the Earth, usually called the yaw axis Z, an axis perpendicular to the plane of the orbit, usually called the pitch axis Y, and an axis perpendicular to the first two usually called the roll axis X. These axes together form a direct trihedron (X, Y, Z). In the case of an equatorial or near-equatorial orbit, the pitch axis Y is at least approximately parallel to the NORTH-SOUTH direction and if the orbit is a circular, geostationary orbit, the roll axis is tangential to the orbit and in the same direction as the instantaneous speed vector of the satellite in its orbit.
The invention is aimed at any satellite in terrestrial orbit using at least one terrestrial sensor to measure roll and pitch angles and a star sensor to measure roll and yaw angles. There is therefore redundancy in respect of the roll measurement.
Satellites wholly or partially stabilized on three-axes in practice have:
a fixed kinetic moment near Y achieved by a pitch momentum wheel or by rotation of part of the satellite, for example, or PA1 an orientable kinetic moment near Y with one degree of freedom provided by, for example, a pitch momentum wheel and a yaw reaction wheel or two momentum wheels in a V configuration about the pitch axis and a yaw reaction wheel or a single momentum wheel on a single-axis pivot in the roll/yaw plane, or PA1 an orientable kinetic moment near Y with multiple degrees of freedom provided, for example, by a single momentum wheel on multiple pivots in the plane, or PA1 a weak (possibly null) kinetic moment of any orientation provided by, for example, three reaction wheels. PA1 the kinetic moment is cancelled by braking the wheels, PA1 Sun pointing mode is selected, PA1 Earth pointing mode is selected, PA1 the wheels are run up to speed. PA1 the reacquisition time is from one to eight hours minimum; PA1 the procedure for capturing the Earth encompasses the followings stages: the wheels are stopped to cancel the kinetic moment, the satellite is rotated about the X axis to search the Sun, the satellite is rotated about the Y axis until the Sun faces +X, the satellite is rotated about the X axis facing the Sun pending the command to search the Earth (at this stage the sequencing is done manually from the ground and the reacquisition time depends on how soon the appropriate decision is taken by the ground personnel), the satellite is rotated about an intermediate axis facing the Sun, the satellite may be rotated about the Z axis, depending on its position in its orbit and the sensors used, and the wheels are run up to speed by manual sequencing; and no star sensor is provided. PA1 Phase a: if the results of tests 1 and 2 are positive, the satellite attitude is controlled in roll, yaw and pitch to capture the Earth and the star and the roll information supplied by the Earth and star sensors is tested for consistency. If the result of the consistency test is positive a normal attitude control mode is selected; if not, processing continues with phase (b) set forth below. PA1 Phase b: if the result of test 1 is positive and the result of test 2 is negative the attitude of the satellite is controlled in roll and in pitch to capture the Earth and the satellite is caused to rotate about the yaw axis until the result of test 2 is positive; the attitude of the satellite is controlled to capture the star and the consistency test of phase (a) is carried out. PA1 Phase c: if the result of test 1 is negative and the result of test 2 is positive, the attitude of the satellite is controlled in roll and yaw to capture the sensed star and the pitch rotation speed is reversed for at most a given time; if the result of test 1 is then positive, the attitude of the satellite is controlled to capture the Earth and the star and the consistency test of phase (a) is carried out; if the result of test 1 is not positive at the end of the given time, Sun acquisition mode is selected. PA1 Phase d: if the results of tests 1 and 2 are negative the roll and yaw speeds are cancelled and the pitch speed is reversed for at most a given time; if the result of test 1 becomes positive test 2 is run; if the result of test 2 remains negative the phase (b) processing continues; otherwise the consistency test of phase (a) is carried out; if the result of test 1 is not positive at the end of the given time, Sun acquisition mode is selected.
Satellite attitude control concepts using a terrestrial sensor and a star sensor are already known and form the subject matter of the patent FR-2.522.614 (inventors Guy MOUILHARAT, Paul A. DUCHON, Jean-Michel A. GUILBERT and Andre A. ROLFO of the CENTRE NATIONALE D'ETUDES SPATIALES) for an "Equatorial orbit satellite configuration with improved solar means" and the patent FR-2.637.565 (inventor Patrick MAUTE of AEROSPATIALE) for a "Three-axis active control system for a geostationary satellite".
The patent FR-2.522.614 concerns a satellite having a platform stabilized to face the Sun using measurements provided by a solar sensor and a star sensor and a payload rotatable about a NORTH-SOUTH axis relative to the platform and stabilized to face the Earth using measurements provided by a terrestrial sensor. The solar and stellar sensors on the one hand and the terrestrial sensor on the other hand are therefore involved in separate stabilization of separate parts of the satellite. There is no particular provision for saving the mission in the event of loss of attitude.
The document FR-2.637.565 describes various attitude control modes (normal mode, station-keeping mode, survival mode and apogee maneuver mode) using terrestrial, solar and stellar sensors. The survival (or save) mode entails pointing a given axis in the roll/yaw plane towards the Sun.
The use of a star sensor to shorten nominal attitude reacquisition by a three-axis stabilized satellite following a fault condition has also been considered in the patent EP-0.338.687 (inventor Nicholas F. MATTHEWS of BRITISH AEROSPACE) for a "Method of and apparatus for returning an earth orbiting spacecraft to an earth pointing attitude after displacement therefrom"; reacquisition is obtained from a configuration in which the satellite is pointed at the Sun, as in the document FR-2.637.565.
The conventional attitude reacquisition procedure for three-axis stabilized satellites is as follows:
The Sun pointing mode entails searching the Sun and then pointing one of the satellite axes towards the Sun and causing the satellite to rotate slowly (typically at 0.5.degree. /s) about this axis. The axis pointed towards the Sun is generally chosen so that the Sun illuminates the solar panels to generate electrical power for the satellite so that its equipment functions correctly. This enables the satellite to remain in this attitude for long periods without risk of damage, although the mission is obviously interrupted. The rotation axis chosen is therefore usually different from the kinetic moment axis, so that it is necessary to cancel the kinetic moment of the satellite before activating this mode. Otherwise the precession imposed on the kinetic moment would require frequent thruster operations and therefore unacceptable consumption of propellant.
The Earth pointing mode entails searching the Earth by rotating the satellite about a satellite axis directed towards the Sun; this axis is in practice different from the Sun pointing mode axis and is chosen so that the field of view of the Earth sensor must intercept the Earth. Once the Earth has been captured, the satellite may be rotated about the yaw axis to return it to its reference attitude.
When this type of procedure is used satellite attitude reacquisition takes at least an hour and may take several hours, even several tens of hours, if attitude is lost in an area of the orbit where it is not possible to have the Sun and the Earth simultaneously in the fields of view of their respective sensors or if a decision by the satellite control station personnel is delayed.
This type of method, routinely used for its simplicity of concept and its reliability, is the subject matter of the document FR-2407860 inter alia in which:
In order to reduce the reacquisition time the patent FR-2649809 proposes to start from the Sun pointing mode and to search the Earth using measurements provided by an additional solar sensor. This procedure enables reacquisition by using the Sun pointing mode at any point in the orbit so as to limit its duration to about one hour provided that the personnel of the satellite control station is available.
An identical result is achieved by the patent EP-0338687 which uses a star sensor. This patent proposes to search the star using the same procedure as is used to search the Earth in the patent FR-2407860. As in the patent FR-2649809, the benefit is that Earth acquisition can be carried out anywhere in the orbit. In these two cases the procedure for searching the Earth can only be undertaken from the Sun pointing mode.
Unlike these three patents, the invention proposes to carry out reacquisition faster (15 to 30 minutes maximum instead of one hour minimum), automatically (no action is required of the ground personnel to load into the onboard software the rotation axis used to capture the Earth or the star), without using a solar sensor, without manual action from the ground to sequence the reacquisition process, with no complicated star recognition algorithm, and avoiding the need to cancel the kinetic moment and so minimizing the consumption of propellant (the wheels continue to rotate at the same speeds, so that it is not necessary to use the thrusters to counter the torque from the wheels when they are braked and then run up to speed again).
Fast reacquisition concepts economical in propellant have been used on satellites having no star detector. Mention may be made of the TVSAT/TDF series or the patent FR-2.620.243 (inventor Ernst BRUDERLE of MESSERSCHMITT-BOLKOW-BLOHM GmBh) for a "Method of reacquisition of the pitch position of a terrestrial satellite"; however, as the title indicates, this process is limited to reacquisition of the attitude of the satellite in the roll/yaw plane only, it being assumed that the orientation of the pitch axis has remained correct.
An object of the invention is to provide a satellite adapted to be three-axis stabilized with an automatic save mode (requiring no action from the ground) which makes it possible in most cases of loss of attitude (not just in the roll/yaw plane) to return the satellite to its nominal attitude within a much shorter time (typically less than thirty minutes) than the prior art methods, with no significant penalty in terms of cost, mass or reliability and requiring only a moderate consumption of propellant. The invention is, therefore, directed to a fast attitude reacquisition procedure which has a sufficiently high probability of success that the risk of having to use an intermediate attitude in which the satellite is pointed towards the Sun (a procedure that is costly in terms of time and/or propellant) becomes negligible in practice.